Production methods

The production of methanol usually consists of three basic steps independent of feedstock material: synthesis gas preparation, methanol synthesis and methanol purification.

In order to properly understand the challenges of different processes to produce the synthesis gas, we first need to understand the process from synthesis gas to methanol, the methanol synthesis. In essence the process consists of the three following equations:

All three equations are reversible and thus the process conditions regarding temperature, pressure and synthesis gas mixture are important to control. It can also be noted that equation (1) and (2) are exothermic, i.e. the processes produce heat and require cooling. Some heat is normally recovered and used for other parts of the synthesis.

While it was originally believed that the main process that produce methanol was the reaction between carbon monoxide and hydrogen (equation 1) it is now understood that carbon dioxide is just as important in the synthesis process. CO2 even used to be scrubbed from the reactant mixture but a scrubber failure at Imperial Chemical Industries (ICI) with a resulting increase of methanol production showed that CO2 was active and important in the reaction. Subsequent studies has shown that it is mainly the CO2 that is converted to methanol while CO act as an reducing agent for oxygen at the surface of the catalyst [4].

Equation (3) describes the reverse water gas shift reaction that produces carbon monoxide from carbon dioxide and hydrogen. The carbon monoxide then reacts with hydrogen to produce methanol (equation 2). Equation 2 is actually merely the sum of (1) and (3) [2].

To synthesise methanol, not only is a specific H2/CO ratio of 2 in the synthesised gas needed but also a (H2-CO2)/(CO+CO2) ratio, called stoichiometric number, equal to or slightly above 2 [3].

In contrast to ethanol, methanol can rather easily be produced from virtually all biomass such as wood, algae, agricultural waste and municipal waste through gasification. Production from biomass does however offer challenges that need to be addressed especially regarding the cost of production. Due to the composition of biomass the production plants inquire high capital investment costs and has lower energy conversion efficiency compared to natural gas and coal [7].

The conventional method to produce methanol from biomass is through gasification of the feedstock material. An attractive alternative is enzymatic conversion, although most research in this area is currently focused on ethanol production. Another alternative mainly for sea growing plants like macro- and microalgae as well as water hyacinth and cattail etc. is anaerobic digestion that produces methane that could be used in the same way as natural gas. As with enzymatic conversion the methods is still in need of further research and development before large scale commercial implementation is possible[2].

The gasification process of biomass is similar to the synthesis gas process from coal. For gasification of biomass the feedstock is first dried and pulverized. The moist content should generally be no higher than 15-20 wt%. The first step in a two-step gasification process is called pyrolysis, or destructive distillation. The dried biomass is heated to 400-600 °C in an oxygen deficient environment to prevent complete combustion. Carbon monoxide, carbon dioxide, hydrogen, methane as well as water and volatile tars are released. The remaining biomass (≈10–25 wt%),called charcoal. is further reacted with oxygen at high temperature (1300-1500 °C) to produce mainly carbon monoxide.

The synthesis gas produced from the pyrolysis and charcoal conversion is purified before the methanol synthesis. Compared to coal biomass consists of much less sulphur but the tar content offers operational challenges as it condense easily in pipes, filters and boilers. This can to an extent be controlled by choosing the right operational pattern and technique according to the composition of the available biomass. A one step partial oxidation process is an attractive alternative but the technical challenges have so far prevented large scale operation.

Production from biomass is possible at small scale but as with natural gas and coal large scale production is preferred due to the high system costs. The logistical challenges for a biomass plant are however great due to the lower energy content in biomass compared to natural gas and coal implying a large demand of feedstock material. The quantities needed to feed a 2500 MTPD plant is estimated to 1.5 million ton biomass per year [2] with put large strains on collection, transportation and storage of biomass and might be one of the largest hurdles towards the construction on mega sized plants [7].

An alternative that has been proposed to solve or ease the transportation demands is to first convert the biomass to bio-crude through fast pyrolysis. Dried and atomized biomass is quickly heated to about 400-600 °C in atmospheric pressure and then quenched to avoid cracking [2]. The result is a black liquid called bio crude that can be transported more easily.